The present invention relates to apparatus for reducing the interference caused by ghosting of TV images in TV receivers.
Ghosting results from the delay in the reflected wave or defracted wave caused by geographical features such as mountains and buildings during propagation of the broadcast wave. This manifests itself as a time delay in the propagation of the modulated signal of the ghost wave with respect to that of the direct wave (the desired broadcast wave). This delay causes a phase difference between the ghost wave and the direct wave at the receiver. Because the TV signal is a VSB (vestigial side-band modulation) signal and causes waveform distortion when the signal having different carrier phase from the phase of the carrier of the direct wave is detected (demodulated), the ghost cannot be cancelled by delaying the detected direct signal only.
In conventional one method (IEEE Trans., Vol. CE-23 No. 3, August 1977); of cancelling a ghost having a different phase, video detection is performed by two phase detector circuits and the first detector phase is adjusted to the phase of the carrier of the direct wave and the second detector phase is adjusted to the phase which reverses to the phase of the carrier of the ghost. In this system, it is necessary to control beforehand the phase of a second detector depending upon the phase of a particular ghost wave. Accordingly, since the second detected phase cannot be detected when two or more ghosts are formed, it is impossible to cancel a plurality of ghost waves. In this method, and also other conventional ghost cancelling methods, it is necessary to set-up the circuit conditions manually to give the no ghost condition (cancelled condition), that depends on the phase, amplitude ratio and delay time of each ghost. Here, these parameters (phase, amplitude ratio and delay time) are different for each ghost and vary between receiving channels. Moreover, the phase of the ghost varies in time. In order to effectively cancel the ghosts, it is necessary to easily follow the variation of the phase of ghost. When a plurality of ghosts are found, it is necessary to set the condition for cancelling the ghosts regardless of the phase of ghosts. In the automatic operation, it is important to be able to follow variation of the phase of ghosts.